The present invention is concerned with the problem of providing coated paper which can be used in sheet form or in the form of a bag or other container for heating articles in a microwave oven. Many products have been proposed for browning, searing or otherwise heating the surface of an article within a microwave oven. These prior products can be divided into three groups: rigid; semi-flexible or having limited flexibility; and flexible. Rigid heating containers are exemplified by the following U.S. Pat. Nos. 4,266,108; 4,184,061; 4,450,334; 4,398,077; 4,166,208; 4,158,760; and 4,320,274. Among these, U.S. Pat. No. 4,266,108 describes a reflective metal plate such as an aluminum plate to which a layer of lossy materials, e.g. magnetic oxides known as ferrites, have been applied. These materials are bonded to the reflective metal plate which can be aluminum by means of an intermediate layer containing a binder on an air gap. U.S. Pat. No. 4,184,061 describes a glass ceramic browning vessel with a metallic oxide coating on its lower surface. U.S. Pat. No. 4,450,334 is similar, except that in this case a plastic layer containing a ferrite is applied to the bottom surface of an aluminum dish. In U.S. Pat. No. 4,398,077 a ceramic or glass dish is described having a resistive film 14 of tin oxide applied to its lower surface. Both of U.S. Pat. Nos. 4,166,208 and 4,158,760 describe rigid conical containers formed from plastic. The lower end of each cone is in contact with the support member such as a block of plastic which is made lossy by the inclusion of metal or carbon particles. U.S. Pat. No. 4,320,270 describes a cooking utensil in the form of a dielectric dish, e.g., glass or plastic, having a metal layer extending through it.
Among references describing flexible packaging materials is U.S. Pat. No. 4,190,757 which describes a supporting substance in the form of aluminum foil which may be as little as one mil in thickness to which a paint-like layer of a ferrite or other lossy material is applied as a coating. For example, a wet mixture of taconite, sand and sodium silicate are blended and applied by brushing and rolling the composition onto a sheet of three mil aluminum to a thickness of 0.03 inches. The resulting laminate is fairly thick (over 30 mils in thickness) and is difficult to handle with automated roll stands and other equipment used for winding, rolling, cutting, transferring and forming sheet material into packages. Moreover, the laminate is heavy and the ferrite coating sometimes tends to flake off when the underlying aluminum sheet is bent or flexed.
U.S. Pat. Nos. 4,267,420 and 4,230,924 describe the use of thin carrier sheets comprising flexible polyester plastic films or laminates of polyester and polyethylene to which a thin semiconductive metal coating is applied. The carrier sheet can, if desired, be bonded to paperboard. U.S. Pat. Nos. 4,553,010, 4,555,605, 4,641,005, 4,678,882 and 4,735,513 show other coated sheets and laminates in which a microwave interactive material is supported upon or dispersed within a carrier sheet formed from plastic.
By contrast with the prior art, a major objective of this invention is to find an effective means for applying microwave interactive coatings to a paper or paperboard backing, for convenience referred to herein simply as "paper", without the use of a separate sheet of metal foil or plastic film as a support or carrier for the interactive layer. Thus, in accordance with the prior art, an interactive substance such as metal, metal oxide, carbon, etc., is either deposited upon a separate plastic carrier sheet, e.g. polyester, or distributed within a film of plastic, e.g. polyethylene or polyethylene tetrathalate, which is in turn bonded to a paper backing sheet. In the work leading to the development of the present invention it was found that when a microwave interactive material, such as an electrodeposited metal layer or carbon, is applied to paper directly there is a tendancy for sparking, popping, arcing and burning to occur when the sheet is exposed to microwave energy. This is of course unacceptable.
One major objective of the invention is to find a way to eliminate the separate plastic sheet used previously as a carrier for the microwave interactive material. Another object is to provide a novel coated sheet material having a cellulose backing coated with a microwave interactive material in such a way that the interactive material will uniformly heat a product, e.g. a food packaged in the sheet material without causing excessive arcing, sparking, popping, scorching or burning of the cellulose sheet material when exposed to microwave energy in a microwave oven.
It is a more specific objective of the present invention to provide an improved cellulose sheet suited for packaging purposes and for producing heat in a microwave oven that does not require a separate plastic sheet functioning as a carrier for microwave interactive material that has to be laminated to the cellulose sheet but which has the following characteristics and advantages:
(a) the ability to absorb microwave energy and transfer the absorbed energy to products in a microwave oven during a heating process without sparking, shrinking, burning, shriveling or disintegrating; PA1 (b) the ability to perform satisfactorily although portions of the sheet extend away from and out of heat transfer relationship with the product being heated; PA1 (c) the ability to carry out heating in certain selected areas and not in others; PA1 (d) the provision of areas of any selected shape such as rectangular, triangular, hexagonal, circular areas, etc., that are capable of heating while other surrounding areas of a flexible sheet have no heating function. PA1 (e) provisions which make possible printing all coating layers, including microwave interactive material, onto a paper backing using a commercially available printing press.
These and other more detailed and specific objects and advantages of the invention will become apparent in view of the following detailed description and accompanying drawings which set forth by way of example certain illustrative embodiments of the invention.